Because of the way resources are divided around the word and way too many other things to go into in the size of a reddit post, industry in general is cheaper in some places than others - and to bring ALL industry internal to your own country to prevent trade or reliance on any other foreign influence (Isolationist), would be too expensive. So expensive that no one could afford to pay for it.

Some things just don't exist in land you control - which has been a huge factor in conflicts in the past. The reason things have gone the way they have, is that people in general have become smarter, relationships have become smoother and trade agreements have benefited each country as a net positive.

Unfortunately, people are forgetting what it was like to be isolationist - how difficult it was for the populace and WHY they had to stop being isolationists.